1. Field of the Invention
The present invention relates to a carrier for two-component electrophotographic developer used in copying machines and printers and to an electrophotographic developer containing the carrier.
2. Description of Related Art
A two-component developer used for developing an electrostatic latent image in electrophotography comprises a toner and a carrier. The carrier is mixed and agitated with the toner in a development box to give a desired charge quantity to the toner and carries the charged toner onto an electrostatic latent image formed on a photosensitive material (photoreceptor) to form a toner image.
The carrier remains on the magnet and is returned to the development box where it is again mixed and agitated with fresh toner particles for repeated use.
In order to maintain high image quality over a service life of a developer in a stable manner, the carrier is required to have stable characteristics over the life.
To meet the requirement, carriers comprising a core and a resin coat have been proposed. Because a developer is always under the stress of collisions among the particles or with the wall of a development box or a photoreceptor, etc. during the service life, the toner adheres to the surface of the carrier particles, that is, so-called spent phenomenon. Further, the resin coat comes off the core by the collisions, which results in variations of charging characteristics, resistivity characteristics and fluidity. As a result, it has been difficult to maintain the initial image quality in a stable manner.
Conventional coating resins, such as a styrene-acrylate copolymer, a styrene-butadiene copolymer, and a polyurethane resin, having a high surface tension, are liable to have toner particles adhere to increase the resistivity. This has been a cause of deterioration of image quality such as fog, i.e., background stains with a toner.
Fluorocarbon resins, while relatively effective in preventing toner adhesion, have low film strength and poor adhesion to the core and therefore tend to separate from the core, which results in reduction of resistivity, failing to maintain the initial image quality.
Silicone resins having a low surface tension have been proposed as a countermeasure against toner adhesion. However, silicone resins per se are apt to be worn, failing to keep electrical resistance, chargeability, fluidity, and the like over the long service life and to maintain the initial image quality.
In order to stabilize the charging characteristics of silicone resins, Japanese Patent Laid-Open Nos. 76754/85 and 284775/86 suggest addition of an organotin compound. The organotin compound accelerates curing of a silicone resin to form a very hard coating film, but the coating film is so brittle to impacts that it comes off considerably due to the stress in a development box to cause large variations in chargeability and resistivity, failing to maintain the initial image quality.
Japanese Patent Laid-Open Nos. 168056/83 and 204666/92 propose introduction of a quaternary ammonium salt into a resin skeleton for the purpose of chargeability maintenance. Although the techniques disclosed are effective in suppressing the change of chargeability with environmental changes, they did not succeed in improving the film strength, failing to maintain the initial image quality over a long service life, either.
Additionally, a resin-coated carrier generally has a high resistivity, which makes it difficult to obtain a sufficient image density and also leads to the liability to cause fogging. In order to overcome these problems as well as the problem that the resistivity is liable to change during a long service life, Japanese Patent Laid-Open No. 126843/81 teaches a method for controlling the initial electrical resistance and stabilizing the resistance characteristics during the service life, which comprises adding an electrically conducting agent to a coating resin.
However, acrylic resins or fluorocarbon resins even containing an electrically conducting agent still fail to maintain the initial image quality not only because of the above-mentioned disadvantage but also because they have poor capability of holding the conducting agent and easily release the conducting agent while use.
Further, Japanese Patent Laid-Open No. 204643/86 proposes addition of an electrically conducting agent to a silicone resin. However, an organotin compound or titanium compound, which is generally used as a curing catalyst for silicone resins, is so powerful that the curing rate is too high to allow the conducting agent to be sufficiently incorporated into a silicone resin before completion of curing. As a result, the resulting cured silicone resin cannot hold the conducting agent certainly, easily releasing the conducting agent under the stress in a development box to cause a drastic change in resistivity, which rather accelerates deterioration of the image characteristics.